Don't re-capture backtraces when propagating traps through host frames (#5049)

* Add a benchmark for traps with many Wasm<-->host calls on the stack

* Add a test for expected Wasm stack traces with Wasm<--host calls on the stack when we trap

* Don't re-capture backtraces when propagating traps through host frames

This fixes some accidentally quadratic code where we would re-capture a Wasm
stack trace (takes `O(n)` time) every time we propagated a trap through a host
frame back to Wasm (can happen `O(n)` times). And `O(n) * O(n) = O(n^2)`, of
course. Whoops. After this commit, it trapping with a call stack that is `n`
frames deep of Wasm-to-host-to-Wasm calls just captures a single backtrace and
is therefore just a proper `O(n)` time operation, as it is intended to be.

Now we explicitly track whether we need to capture a Wasm backtrace or not when
raising a trap. This unfortunately isn't as straightforward as one might hope,
however, because of the split between `wasmtime::Trap` and
`wasmtime_runtime::Trap`. We need to decide whether or not to capture a Wasm
backtrace inside `wasmtime_runtime` but in order to determine whether to do that
or not we need to reflect on the `anyhow::Error` and see if it is a
`wasmtime::Trap` that already has a backtrace or not. This can't be done the
straightforward way because it would introduce a cyclic dependency between the
`wasmtime` and `wasmtime-runtime` crates. We can't merge those two `Trap`
types-- at least not without effectively merging the whole `wasmtime` and
`wasmtime-runtime` crates together, which would be a good idea in a perfect
world but would be a *ton* of ocean boiling from where we currently are --
because `wasmtime::Trap` does symbolication of stack traces which relies on
module registration information data that resides inside the `wasmtime` crate
and therefore can't be moved into `wasmtime-runtime`. We resolve this problem by
adding a boolean to `wasmtime_runtime::raise_user_trap` that controls whether we
should capture a Wasm backtrace or not, and then determine whether we need a
backtrace or not at each of that function's call sites, which are in `wasmtime`
and therefore can do the reflection to determine whether the user trap already
has a backtrace or not. Phew!

Fixes #5037

* debug assert that we don't record unnecessary backtraces for traps

* Add assertions around `needs_backtrace`

Unfortunately we can't do

    debug_assert_eq!(needs_backtrace, trap.inner.backtrace.get().is_some());

because `needs_backtrace` doesn't consider whether Wasm backtraces have been
disabled via config.

* Consolidate `needs_backtrace` calculation followed by calling `raise_user_trap` into one place

benches/trap.rs

@@ -9,6 +9,7 @@ fn bench_traps(c: &mut Criterion) {
     bench_multi_threaded_traps(c);
     bench_many_modules_registered_traps(c);
     bench_many_stack_frames_traps(c);
+    bench_host_wasm_frames_traps(c);
 }
 
 fn bench_multi_threaded_traps(c: &mut Criterion) {
@@ -159,6 +160,66 @@ fn bench_many_stack_frames_traps(c: &mut Criterion) {
     group.finish()
 }
 
+fn bench_host_wasm_frames_traps(c: &mut Criterion) {
+    let mut group = c.benchmark_group("host-wasm-frames-traps");
+
+    let wat = r#"
+        (module
+            (import "" "" (func $host_func (param i32)))
+            (func (export "f") (param i32)
+                local.get 0
+                i32.eqz
+                if
+                    unreachable
+                end
+
+                local.get 0
+                i32.const 1
+                i32.sub
+                call $host_func
+            )
+        )
+    "#;
+
+    let engine = Engine::default();
+    let module = Module::new(&engine, wat).unwrap();
+
+    for num_stack_frames in vec![20, 40, 60, 80, 100, 120, 140, 160, 180, 200] {
+        group.throughput(Throughput::Elements(num_stack_frames));
+        group.bench_with_input(
+            BenchmarkId::from_parameter(num_stack_frames),
+            &num_stack_frames,
+            |b, &num_stack_frames| {
+                b.iter_custom(|iters| {
+                    let mut store = Store::new(&engine, ());
+                    let host_func = Func::new(
+                        &mut store,
+                        FuncType::new(vec![ValType::I32], vec![]),
+                        |mut caller, args, _results| {
+                            let f = caller.get_export("f").unwrap();
+                            let f = f.into_func().unwrap();
+                            f.call(caller, args, &mut [])?;
+                            Ok(())
+                        },
+                    );
+                    let instance = Instance::new(&mut store, &module, &[host_func.into()]).unwrap();
+                    let f = instance
+                        .get_typed_func::<(i32,), (), _>(&mut store, "f")
+                        .unwrap();
+
+                    let start = std::time::Instant::now();
+                    for _ in 0..iters {
+                        assert!(f.call(&mut store, (num_stack_frames as i32,)).is_err());
+                    }
+                    start.elapsed()
+                });
+            },
+        );
+    }
+
+    group.finish()
+}
+
 fn module(engine: &Engine, num_funcs: u64) -> Result<Module> {
     let mut wat = String::new();
     wat.push_str("(module\n");

crates/runtime/src/component/transcode.rs

@@ -86,7 +86,12 @@ mod trampolines {
                     });
                     match result {
                         Ok(Ok(ret)) => transcoders!(@convert_ret ret _retptr $($result)?),
-                        Ok(Err(err)) => crate::traphandlers::raise_trap(err.into()),
+                        Ok(Err(err)) => crate::traphandlers::raise_trap(
+                            crate::traphandlers::TrapReason::User {
+                                error: err,
+                                needs_backtrace: true,
+                            },
+                        ),
                         Err(panic) => crate::traphandlers::resume_panic(panic),
                     }
                 }

crates/runtime/src/libcalls.rs

@@ -165,10 +165,20 @@ pub mod trampolines {
     }
 }
 
-unsafe fn memory32_grow(vmctx: *mut VMContext, delta: u64, memory_index: u32) -> Result<*mut u8> {
+unsafe fn memory32_grow(
+    vmctx: *mut VMContext,
+    delta: u64,
+    memory_index: u32,
+) -> Result<*mut u8, TrapReason> {
     let instance = (*vmctx).instance_mut();
     let memory_index = MemoryIndex::from_u32(memory_index);
-    let result = match instance.memory_grow(memory_index, delta)? {
+    let result =
+        match instance
+            .memory_grow(memory_index, delta)
+            .map_err(|error| TrapReason::User {
+                error,
+                needs_backtrace: true,
+            })? {
             Some(size_in_bytes) => size_in_bytes / (wasmtime_environ::WASM_PAGE_SIZE as usize),
             None => usize::max_value(),
         };

crates/runtime/src/traphandlers.rs

@@ -95,8 +95,11 @@ pub unsafe fn raise_trap(reason: TrapReason) -> ! {
 /// Only safe to call when wasm code is on the stack, aka `catch_traps` must
 /// have been previously called. Additionally no Rust destructors can be on the
 /// stack. They will be skipped and not executed.
-pub unsafe fn raise_user_trap(data: Error) -> ! {
-    raise_trap(TrapReason::User(data))
+pub unsafe fn raise_user_trap(error: Error, needs_backtrace: bool) -> ! {
+    raise_trap(TrapReason::User {
+        error,
+        needs_backtrace,
+    })
 }
 
 /// Raises a trap from inside library code immediately.
@@ -138,7 +141,12 @@ pub struct Trap {
 #[derive(Debug)]
 pub enum TrapReason {
     /// A user-raised trap through `raise_user_trap`.
-    User(Error),
+    User {
+        /// The actual user trap error.
+        error: Error,
+        /// Whether we need to capture a backtrace for this error or not.
+        needs_backtrace: bool,
+    },
 
     /// A trap raised from Cranelift-generated code with the pc listed of where
     /// the trap came from.
@@ -149,6 +157,22 @@ pub enum TrapReason {
 }
 
 impl TrapReason {
+    /// Create a new `TrapReason::User` that does not have a backtrace yet.
+    pub fn user_without_backtrace(error: Error) -> Self {
+        TrapReason::User {
+            error,
+            needs_backtrace: true,
+        }
+    }
+
+    /// Create a new `TrapReason::User` that already has a backtrace.
+    pub fn user_with_backtrace(error: Error) -> Self {
+        TrapReason::User {
+            error,
+            needs_backtrace: false,
+        }
+    }
+
     /// Is this a JIT trap?
     pub fn is_jit(&self) -> bool {
         matches!(self, TrapReason::Jit(_))
@@ -157,7 +181,7 @@ impl TrapReason {
 
 impl From<Error> for TrapReason {
     fn from(err: Error) -> Self {
-        TrapReason::User(err)
+        TrapReason::user_without_backtrace(err)
     }
 }
 
@@ -381,7 +405,21 @@ impl CallThreadState {
     }
 
     fn unwind_with(&self, reason: UnwindReason) -> ! {
-        let backtrace = self.capture_backtrace(None);
+        let backtrace = match reason {
+            // Panics don't need backtraces. There is nowhere to attach the
+            // hypothetical backtrace to and it doesn't really make sense to try
+            // in the first place since this is a Rust problem rather than a
+            // Wasm problem.
+            UnwindReason::Panic(_)
+            // And if we are just propagating an existing trap that already has
+            // a backtrace attached to it, then there is no need to capture a
+            // new backtrace either.
+            | UnwindReason::Trap(TrapReason::User {
+                needs_backtrace: false,
+                ..
+            }) => None,
+            UnwindReason::Trap(_) => self.capture_backtrace(None),
+        };
         unsafe {
             (*self.unwind.get()).as_mut_ptr().write((reason, backtrace));
             wasmtime_longjmp(self.jmp_buf.get());

crates/wasmtime/src/component/func/host.rs

@@ -1,7 +1,7 @@
 use crate::component::func::{Memory, MemoryMut, Options};
 use crate::component::storage::slice_to_storage_mut;
 use crate::component::{ComponentNamedList, ComponentType, Lift, Lower, Type, Val};
-use crate::{AsContextMut, StoreContextMut, ValRaw};
+use crate::{AsContextMut, StoreContextMut, Trap, ValRaw};
 use anyhow::{anyhow, bail, Context, Result};
 use std::any::Any;
 use std::mem::{self, MaybeUninit};
@@ -265,7 +265,7 @@ fn validate_inbounds<T: ComponentType>(memory: &[u8], ptr: &ValRaw) -> Result<us
 unsafe fn handle_result(func: impl FnOnce() -> Result<()>) {
     match panic::catch_unwind(AssertUnwindSafe(func)) {
         Ok(Ok(())) => {}
-        Ok(Err(e)) => wasmtime_runtime::raise_user_trap(e),
+        Ok(Err(e)) => Trap::raise(e),
         Err(e) => wasmtime_runtime::resume_panic(e),
     }
 }

crates/wasmtime/src/func.rs

@@ -11,9 +11,8 @@ use std::pin::Pin;
 use std::ptr::NonNull;
 use std::sync::Arc;
 use wasmtime_runtime::{
-    raise_user_trap, ExportFunction, InstanceHandle, VMCallerCheckedAnyfunc, VMContext,
-    VMFunctionBody, VMFunctionImport, VMHostFuncContext, VMOpaqueContext, VMSharedSignatureIndex,
-    VMTrampoline,
+    ExportFunction, InstanceHandle, VMCallerCheckedAnyfunc, VMContext, VMFunctionBody,
+    VMFunctionImport, VMHostFuncContext, VMOpaqueContext, VMSharedSignatureIndex, VMTrampoline,
 };
 
 /// A WebAssembly function which can be called.
@@ -1887,7 +1886,7 @@ macro_rules! impl_into_func {
 
                     match result {
                         CallResult::Ok(val) => val,
-                        CallResult::Trap(trap) => raise_user_trap(trap),
+                        CallResult::Trap(err) => Trap::raise(err),
                         CallResult::Panic(panic) => wasmtime_runtime::resume_panic(panic),
                     }
                 }

crates/wasmtime/src/trampoline/func.rs

@@ -56,7 +56,7 @@ unsafe extern "C" fn stub_fn<F>(
         // call-site, which gets unwrapped in `Trap::from_runtime` later on as we
         // convert from the internal `Trap` type to our own `Trap` type in this
         // crate.
-        Ok(Err(trap)) => wasmtime_runtime::raise_user_trap(trap.into()),
+        Ok(Err(trap)) => Trap::raise(trap.into()),
 
         // And finally if the imported function panicked, then we trigger the
         // form of unwinding that's safe to jump over wasm code on all

crates/wasmtime/src/trap.rs

@@ -252,6 +252,15 @@ impl Trap {
         Trap::new_with_trace(TrapReason::I32Exit(status), None)
     }
 
+    // Same safety requirements and caveats as
+    // `wasmtime_runtime::raise_user_trap`.
+    pub(crate) unsafe fn raise(error: anyhow::Error) -> ! {
+        let needs_backtrace = error
+            .downcast_ref::<Trap>()
+            .map_or(true, |trap| trap.trace().is_none());
+        wasmtime_runtime::raise_user_trap(error, needs_backtrace)
+    }
+
     #[cold] // see Trap::new
     pub(crate) fn from_runtime_box(
         store: &StoreOpaque,
@@ -264,9 +273,14 @@ impl Trap {
     pub(crate) fn from_runtime(store: &StoreOpaque, runtime_trap: wasmtime_runtime::Trap) -> Self {
         let wasmtime_runtime::Trap { reason, backtrace } = runtime_trap;
         match reason {
-            wasmtime_runtime::TrapReason::User(error) => {
+            wasmtime_runtime::TrapReason::User {
+                error,
+                needs_backtrace,
+            } => {
                 let trap = Trap::from(error);
                 if let Some(backtrace) = backtrace {
+                    debug_assert!(needs_backtrace);
+                    debug_assert!(trap.inner.backtrace.get().is_none());
                     trap.record_backtrace(TrapBacktrace::new(store, backtrace, None));
                 }
                 trap
@@ -359,12 +373,15 @@ impl Trap {
     fn record_backtrace(&self, backtrace: TrapBacktrace) {
         // When a trap is created on top of the wasm stack, the trampoline will
         // re-raise it via
-        // `wasmtime_runtime::raise_user_trap(trap.into::<Box<dyn Error>>())`
-        // after panic::catch_unwind. We don't want to overwrite the first
-        // backtrace recorded, as it is most precise.
-        // FIXME: make sure backtraces are only created once per trap! they are
-        // actually kinda expensive to create.
-        let _ = self.inner.backtrace.try_insert(backtrace);
+        // `wasmtime_runtime::raise_user_trap(trap.into::<Box<dyn Error>>(),
+        // ..)` after `panic::catch_unwind`. We don't want to overwrite the
+        // first backtrace recorded, as it is most precise. However, this should
+        // never happen in the first place because we thread `needs_backtrace`
+        // booleans throuch all calls to `raise_user_trap` to avoid capturing
+        // unnecessary backtraces! So debug assert that we don't ever capture
+        // unnecessary backtraces.
+        let result = self.inner.backtrace.try_insert(backtrace);
+        debug_assert!(result.is_ok());
     }
 }
 

tests/all/traps.rs

@@ -69,6 +69,72 @@ fn test_trap_trace() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_trap_through_host() -> Result<()> {
+    let wat = r#"
+        (module $hello_mod
+            (import "" "" (func $host_func_a))
+            (import "" "" (func $host_func_b))
+            (func $a (export "a")
+                call $host_func_a
+            )
+            (func $b (export "b")
+                call $host_func_b
+            )
+            (func $c (export "c")
+                unreachable
+            )
+        )
+    "#;
+
+    let engine = Engine::default();
+    let module = Module::new(&engine, wat)?;
+    let mut store = Store::<()>::new(&engine, ());
+
+    let host_func_a = Func::new(
+        &mut store,
+        FuncType::new(vec![], vec![]),
+        |mut caller, _args, _results| {
+            caller
+                .get_export("b")
+                .unwrap()
+                .into_func()
+                .unwrap()
+                .call(caller, &[], &mut [])?;
+            Ok(())
+        },
+    );
+    let host_func_b = Func::new(
+        &mut store,
+        FuncType::new(vec![], vec![]),
+        |mut caller, _args, _results| {
+            caller
+                .get_export("c")
+                .unwrap()
+                .into_func()
+                .unwrap()
+                .call(caller, &[], &mut [])?;
+            Ok(())
+        },
+    );
+
+    let instance = Instance::new(
+        &mut store,
+        &module,
+        &[host_func_a.into(), host_func_b.into()],
+    )?;
+    let a = instance
+        .get_typed_func::<(), (), _>(&mut store, "a")
+        .unwrap();
+    let err = a.call(&mut store, ()).unwrap_err();
+    let trace = err.trace().expect("backtrace is available");
+    assert_eq!(trace.len(), 3);
+    assert_eq!(trace[0].func_name(), Some("c"));
+    assert_eq!(trace[1].func_name(), Some("b"));
+    assert_eq!(trace[2].func_name(), Some("a"));
+    Ok(())
+}
+
 #[test]
 #[allow(deprecated)]
 fn test_trap_backtrace_disabled() -> Result<()> {